ADA MP1 into a pedal

caspercody · Post by **caspercody** » 21 Nov 2020, 23:52

Making progress today. I took the buffer out for now, and just got the U4 op amp section to work. I removed the 20K resistor, and 20K pot and replaced these with (1) 500K pot. Now this works. 500K might be a little high in value, because this thing will get really overdriven at max on the 500K pot. Plus, do need to ground the 100K resistor otherwise way to much low end.

I used a separate pedal with a buffer at the front of this, which this thing does need a buffer, otherwise sucks the tone out.

Does the buffer in this design really make a difference in the sound? Or would any buffer do?

Optical · Post by **Optical** » 24 Nov 2020, 23:29

That's cool.
Def need a buffer before the gain control - Probably any FET buffer will sound the same, there's no tone shaping going on that I can see.

caspercody · Post by **caspercody** » 25 Nov 2020, 04:03

So the input circuit is more than a buffer, it makes it more treble sounding, and boosts the signal. But it is noisy.

Is this thing kind of trebly sounding, nasal?

caspercody · Post by **caspercody** » 26 Nov 2020, 05:51

The circuit at the beginning (Q6/Q7) appears to be what is called a JFET/BJT feedback amp (or Sziklai pair, compound transistor), this boosts the signal a lot. Just the U4 circuit by itself barely gets the signal to unity gain. Right now I have a circuit of the Q6/Q7, and U4 in a separate pedal which then connects to the tube circuit (in a separate box) and it works, but it is very trebly sounding.

I want to add a pot between the (2) circuits, because I think this will be a better gain control, since the Q6,/Q7 is hitting the U4 circuit wide open. Maybe this is causing the trebly sound. When I add it there is no volume, until you turn to 9 then there is sound. From 9 to 10. Anyone have any ideas how to add a pot to adjust the output from Q6/Q7, and into U4?

Another reason I added this into a pedal, is if I use my VFE Ice Scream in front of the tube circuit, it sounds really nice. And with all the controls on the Ice Scream you can really change up the tone.

But I have worked too much on this project not to get the whole circuit working. Just want to figure out how to adjust output of Q6/Q7 into U4, I think this might work?

Please let me know if you have any ideas?

ATL Strat · Post by **ATL Strat** » 26 Nov 2020, 13:45

You’re doing a great job with this! Just do a simple volume control pot out of Q6/Q7. That would be out of Q6/Q7 into connection 3 of a 1M pot. Connection 2 will go to U4 and connection 3 will go to ground. When looking at the back of the pot with the connectors/terminals on top, #3 is the one on the right.

Keep up the good work!

caspercody · Post by **caspercody** » 26 Nov 2020, 14:30

Thanks for the reply. Unfortunately that is one of the ways I tried hooking it up. I get no volume as I turn up it up, until I get to say 9, then from 9 to 10 is the only range of volume. I also tried connecting the pot by using same connections but connecting pin 1 also to pin 2. Same results with the volume

caspercody · Post by **caspercody** » 26 Nov 2020, 15:19

Okay, I grabbed another pot, just to make sure I was not grabbing the same one every time I try this, and for some dumb luck that pot was bad. It was not bad.

What I did different this time was turn the amp up! The 1M volume pot is connected connection 3 of a 1M pot. Connection 2 will go to U4 and connection 3 will to ground. When I turn the pot from zero to 1 I hear no signal, but a rubbing dull noise, then at 1, I get a low signal. As I turn up the pot I get a clean signal, that does not really go up in volume. When I get to say 8, then volume goes up and slight distortion. There is a very thin line as I turn it up from 8-9 where the volume jumps way up. The distortion is not good. When I switch on the 100K at end of circuit, have a big volume jump, and a lot of bass added. But the pot works the same way as I described.

The only way I have gotten something to work like a gain pot, is to remove the 20K resistor in the U4 stage loop, and connect the end of the 20K pot to Pin2. Then I have a pot that works great, but the sound is very trebly, thin sounding. When I connect it this way, I am not putting in the 1M pot

ATL Strat · Post by **ATL Strat** » 26 Nov 2020, 15:39

Strange. It should actually be fine without the volume pot, so something is off. I’m not sure if you’ve posted an updated schematic, but I would double check your values/components against the attached layout. It won’t show you connections, but it shows values.

Sorry, I guess it would help for me to upload the “key” as well, so you can line the values up to the schematic. Here you go:

ATL Strat · Post by **ATL Strat** » 26 Nov 2020, 19:05

Here’s something that you can try. Below is the schematic for the “Ruby Guitar Amplifier”, which has a simple FET buffer as the input. Maybe try this setup (everything through the volume pot) going into U4. It shouldn’t take more than a minute or two to wire up. More info here:

https://robrobinette.com/How_the_Ruby_Amp_Works.htm

caspercody · Post by **caspercody** » 26 Nov 2020, 20:00

I have tried another buffer before the U4, but the Q6/Q7 stage is a buffer/preamp circuit. It boosts the signal, which is needed before the U4 stage.

Do you have a better picture of the layout showing the device numbers?

ATL Strat · Post by **ATL Strat** » 26 Nov 2020, 20:32

I don’t. I think those are the best copies available. If you click and zoom in, you should be able to see most of them. That being said, some of numbers are a bit tough to read.

caspercody · Post by **caspercody** » 27 Nov 2020, 00:08

VTL.pdf: (46.91 KiB) Downloaded 91 times

Thanks ATL Strat, with that layout showing values I was able to look up the VTL component, and come up with a better idea of how it is wired up. See attached new document. This sounds a lot better now.

Still curious of the value of the resistor in the VTL?? I have attached the VTL PDF I found, can someone look at the attached PDF and let me know what you think for value? I think it might be 400 ohms, but the graphs show 100k? Right now I have the lowest pot value I have, 500 ohms in the circuit. Now I do get a signal out with the gain pot at zero, and it somewhat increases as you turn it up, and from 9 to 10 it still has the huge jump in gain.

Also curious if I need to add U9 to the circuit? Not the VTL LED portion, but if you look at the VTL PDF and the ADA schematic at U5 the bottom resistor in U5 is Pin3 and it has some input from U9, The top resistor in U5 is Pin 4, and Pin 5.

ATL Strat · Post by **ATL Strat** » 27 Nov 2020, 00:50

I didn’t see the VTL pdf attached. Is this the right one:

http://www.farnell.com/datasheets/87361.pdf

ATL Strat · Post by **ATL Strat** » 27 Nov 2020, 01:14

BTW, the schematic shows the earlier Zeta IC’s, not the VTLs that were used later on (which are shown on the layout). The change is mentioned here:

http://www.marshalljmpmodshop.net/ADA-M ... s-1992.pdf

I’ve had units with both - the VTLs are superior.

ATL Strat · Post by **ATL Strat** » 27 Nov 2020, 02:09

BTW, I’m wondering if 12v vs. 15v is messing with things a bit. That’s a 20% drop in voltage.

caspercody · Post by **caspercody** » 30 Nov 2020, 18:07

I am done with this project. I have created the Q6/Q7 input stage, along with the U4, and added U9 into a pedal box. And everything seems to work. However, the tube circuit sounds so much better when using my VFE Ice Scream pedal in front of the tube circuit. The distortion sounds the same, to me, but with the VFE in front I have so much more control over the tone.

ATL Strat · Post by **ATL Strat** » 01 Dec 2020, 15:43

Awesome! Congrats! Do you have a final schematic that others can use as a starting point? Also, do you feel like it sounds like an MP-1?

caspercody · Post by **caspercody** » 01 Dec 2020, 22:24

The tube schematic is in the very first post. The rest of what I used is in the post (6) post up from this one.

I do not have a MP1, so I do not know how it sounds in person. All the video's I see on Youtube seem to be post production changes, so it is hard to determine from the video's what this sounds like.

caspercody · Post by **caspercody** » 01 Dec 2020, 22:35

I ordered a board from alex.hotkits@gmail.com, called the GTO (Great Tone Overdrive). It is a high voltage Soldano GTO Supercharger pedal. I attached the build documents I got from him.

Then using the schematic for the ADA MP1, I changed the GTO schematic to look like the ADA MP1. I ordered most of the parts from Tayda, except the high voltage capacitors and 90 degree connectors. I ordered these parts from Mouser. I got some turret boards from Ebay for the tone stack components. For the tube sockets, the best one's i used was the one 's I got from Alex Hotkits.

I made (11) total different amp pre-amps: Dumble Overdrive special, Peavey 5150 (blue, and red channels), Marshall JCM800, ADA MP1, Orange Rockerverb, Diezel VH4 (channel 3), Fender Vibroverb, Mesa Boogie Mark 5, Mesa Boogie Dual Rectifier, and Soldano SLO100.

ATL Strat · Post by **ATL Strat** » 02 Dec 2020, 20:54

Very cool. Thanks for the info!